The intermetallic compound triniobium tin, Nb.sub.3 Sn, is a type-II metallic superconductor of interest because it has high values of superconducting critical current density in high magnetic fields. Critical current density, J.sub.c, is a value resulting from division of the critical current measured in a magnetic field by the cross-sectional area of the superconductor.
Historically, Nb.sub.3 Sn has been formed using a number of different processes. These include: 1) condensation from the vapor phase; 2) crystallization from the liquid phase; 3) diffusion between one solid phase and one liquid phase; and 4) diffusion between two solid phases. This invention is related to the solid-liquid diffusion method which is now described.
Methods for forming triniobium tin superconductors by liquid-solid diffusion require multiple steps. The general method is well known and a description of this method is given below.
Generally, the first step in forming triniobium tin superconductor is to clean the niobium or niobium based substrate, usually a wire or foil. This is done with a cleaning solution or etchant, such as a mixture of nitric acid, hydrochloric acid, and water. Diluted hydrofluoric acid is also sometimes used for cleaning the substrate. After the substrate is cleaned, oxygen may be added to the surface of the substrate by anodizing the surface electrolytically.
The next three steps involve high temperature heat treatments. The first anneal, as taught by Caslaw in British patent 1,342,726, is used to introduce a desired oxygen content into the niobium substrate. This is accomplished by passing the substrate through a furnace at about 950.degree. C. for about 30 seconds in an atmosphere containing argon and oxygen. However, if the substrate has been previously anodized to form an oxide layer on the surface of the substrate, then the preheat is called a decomposition anneal whereby the substrate is annealed so that the oxide layer diffuses into the body of the substrate. This would be conducted in an oxygen free environment. At this point the niobium substrate may be wound in a coil until further processing or may continue to the tin dip process.
After the preheat, the substrate is dipped in a tin or tin alloy bath, which supplies the tin for the triniobium tin reaction. The operating temperature for the tin bath is between 900.degree.-1100.degree. C.
The tin coating from the bath has a limiting thickness due to the amount of tin needed to further react with the niobium. This thickness is about two to thirty micrometers of tin. The tin thickness is inversely proportional to the viscosity of the tin bath and the temperature of the tin bath, and proportional to the foil speed. Thus, the tin thickness places a lower limit on the substrate speed in the tin dip, which, as an example, is about thirteen centimeters per second for a tin bath at 900.degree.-1000.degree. C.
Subsequently, after the tin coating is applied, the niobium substrate is cooled before being treated with a reaction anneal to react the tin coating and the niobium base metal. During this final anneal, a layer of superconducting triniobium tin is formed on both sides of the niobium substrate. In the reaction anneal, the foil is moved through a furnace at about 1050.degree. C. and the speed of the foil depends on the anneal time and the height of the furnace. For instance, in a five meter furnace for about two hundred seconds at 1050.degree. C., the foil speed is about 2.5 centimeters per second.
During the reaction anneal step, as the tin-coated foil is passed through a vertical furnace in an inert atmosphere, the tin-alloy coating becomes molten and reacts with the niobium substrate to form the superconducting intermetallic, triniobium tin. However, during the reaction the tin will often flow creating tin-depleted regions, areas of exposed triniobium tin, and regions of tin "rivers" or tin "balls". As a result, the depletion of tin on the substrate limits the growth of triniobium tin and subsequently limits the critical current density that can be achieved by the superconductor. Additionally, solder will not wet the exposed triniobium tin surface interface to provide an adequate bond to the copper during subsequent lamination processes. Also, the non-uniform surface condition of the triniobium tin superconductor after the reaction anneal, will create stress concentrators which will fracture the superconducting layer, thus rendering the superconductor useless.
Thus, there is a need for a method to form a continuous triniobium tin superconductor with a uniform surface coating of tin alloy during and after reaction anneal.